Which STI value is considered intelligible?

Which STI value is considered intelligible?

• A. 0.3
• B. 0.7
• C. 0.9
• D. 0.5

Answer: (D)

STI, or Speech Transmission Index, measures how well speech can be understood after it travels through a room or system. It runs from 0 to 1, with higher numbers meaning clearer, more intelligible speech. In practice, about 0.5 is the threshold where speech starts to become intelligible under typical listening conditions; below that, understanding tends to drop off, while values higher than 0.5 indicate progressively better intelligibility. So, the value around 0.5 represents the point at which speech becomes reliably intelligible for listeners. Values like 0.7 or 0.9 are still intelligible and even clearer, but 0.5 marks the lower boundary where intelligibility begins.